1. Field of the Invention
The present invention relates to a radio communication system and particularly relates to a transmission method of a broadcast signal.
2. Description of the Related Art
In general, a receiver in a radio communication system is required to know in advance concrete information (information necessary for communication such as a transmission frequency and synchronization information, for example) of a corresponding transmitter. It is suggested a technology in which transmission of a broadcast signal and the like by a transmitter enables communication even if a receiver does not know part of such concrete information in advance. As a method in which a receiver detects existence of a transmitter of an existing radio communication system and further establishes time synchronization with the transmitter in the system, there is known a method of transmitting a transmission sequence superior in an autocorrelation characteristic continuously in terms of time (for example, see JP-A 2000-341240 (KOKAI)).
According to this method, the transmitter transmits a signal sequence of a given sequence length continuously and the receiver multiplies together the received signal and a signal made by delaying the received signal by the sequence length and subjecting the delayed signal to a complex conjugate computation, whereby a high correlation value can be obtained. Further, a time at which the correlation value is obtained by this method can be regarded as a time when the transmitter transmits the signal, and therefore it is possible to establish time synchronization between the transmitter and the receiver. If the high correlation value cannot be obtained by this method, it can be regarded that such a signal sequence is not transmitted, and it is possible to judge that the transmitter does not exist.
As a means for detecting a transmission signal by such a method, there is known a method in which a matched filter is used. In this method, a signal sequence with high autocorrelation capability is decided in advance between a transmitter and a receiver and the transmitter transmits this signal sequence with a given frequency. The corresponding receiver operates in advance a matched filter whose tap coefficient is this signal sequence and obtains a high signal output if the signal sequence is received, whereby detects existence of the transmitter.
However, in the method in which the same signal sequence is repeatedly transmitted, since the received signal is delayed, the receiver must have a relatively large capacity memory. This is ineffective in terms of design, and becomes a cause of a large scale device configuration, increased power consumption and the like.
In a method in which the matched filter is used, it is required to negotiate the signal sequence in advance, and in addition, the frequency with which the signal sequence is transmitted must be known. Therefore, in a circumstance in which the transmission frequency of the signal sequence is not known, using the method in which the matched filter is used is difficult.